The present invention relates generally to an optical element obtained by configuring a material having satisfactory optical properties represented by refractive-index chromatic dispersion characteristics by molding or forming means into a given shape, for instance, lenses, prisms, filters, refracting optical elements and diffracting optical elements.
So far, chromatic aberrations of dioptric systems have been corrected by combinations of optical materials having varying refractive-index dispersion characteristics. By making a selection from many optical materials having varying refractive indices and varying refractive-index chromatic dispersion characteristics for optical designs, the degree of freedom in the optical designs is so enhanced that chromatic aberration-free, high-precision optical systems can easily be designed.
For such optical materials, there have been developed a variety of optical glasses having varying refractive indices and varying chromatic dispersion characteristics.
FIG. 1 is illustrative of the refractive index vs. chromatic dispersion relations in optical glasses and optical resins used so far as optical materials.
Some available optical glasses have a refractive index of about 1.43 to 2 and an Abbe number vd of about 20 to 95 indicative of refracting-index chromatic dispersion characteristics. On the other hand, synthetic resins that may be used as optical materials are limited in terms of type and narrower than optical glasses in terms of the range of the optical properties to be selected, as shown in FIG. 1, and so optical designs using synthetic resins have many restrictions.
To reduce the weight of taking optical systems such as cameras and improve their performance, optical elements having optically effective surfaces in an aspheric or free-form surface form rather than in a conventional spherical form have now been used. The use of aspheric optical elements is found to make some contributions to performance improvements through reductions in the number of lenses used and removal of spherical aberrations. With cutting and polishing that is a typical method for processing conventional optical elements such as glass lenses, however, it is very difficult to configure aspheric forms, and so this method does not lend itself to mass production. For that reason, as known from JP-A 6-32631, aspheric or other surfaces outside of spherical ones have been processed using molds having a precisely mirror-finished molding surface; optical glasses have been processed by precise press molding and synthetic resins by injection molding, cast molding or the like. Even in those cases, optical materials having a low dispersion, i.e., a large Abbe number vd indicative of low chromatic dispersion characteristics in the visible range must be used for correction of chromatic aberrations in view of optical designs. Thus, optical glasses having a variety of low refractive-index chromatic dispersions have been developed, as known typically from JP-B 4-33740 and JP-A 7-172864.
However, current commercial optical glasses having low refracting-index chromatic dispersions, because of being based on fluorophosphates, cause damage to the precisely processed molding surface of a mold under the influences of glass components when they are subjected to precise press molding at a high temperature at which they soften, making it difficult to maintain the surface precision of the molding surface of the mold. Thus, mass-production of high-quality optical glasses is still difficult.
With the aforesaid fluorophosphate glass, the radius of curvature of the optically effective surface of an optical element must be diminished in view of optical design because its refractive index is low. This causes the thickness deviation to become considerably large, resulting in an increase in the level of difficulty in molding processes. For instance, precise press molding for glasses has a possibility of glass cracking upon drastic deformation given to the softening glass via the molding surface of the mold, and so renders it very difficult to mold and process lenses having too large a thickness deviation.
By contrast, synthetic resins, because of being improved in moldability or formability, and capable of configuring lenses having a large thickness deviation by injection molding or cast molding, facilitate fabrication of lenses, etc. As shown in FIG. 1, however, the synthetic resins are not only narrow in the range of optical properties but also provide an optical element that is poor in environmental stability due to a high rate of thermal expansion and hygroscopic deformation depending on composition. Thus, the synthetic resins have a problem in that they are unsuitable for mass production.
It is therefore an object of the present invention to provide an optical element capable of maintaining high performance, for example, a lens, which is obtained by molding or otherwise forming an optical material that has optical properties in a low chromatic dispersion range extending the range of optical properties, a high degree of freedom in molding or forming as is the case with synthetic resins and moldability or formability suited for mass production, and is superior in environmental stability to synthetic resins used so far in optical applications.